1. Field of the Invention
The invention relates to a circuit for horizontal scanning, referred to as a diode modulator, for use in a video display apparatus, comprising a main semiconductor switching element, with an active terminal and a terminal connected to ground, arranged in parallel with two diodes connected in cascade in a direction opposite to the direct direction of the main switching element, provided with a first branch connected between the active terminal of the main switching element and ground, and consisting of the cascade arrangement of a deflection coil, a first S capacitance and an inductance, and provided with a second branch consisting of the cascade arrangement of an additional S capacitance and a thyristor for activating or deactivating said additional S capacitance.
The invention is, inter alia, usable for activating or deactivating an additional S capacitance for the purpose of correcting the lateral parts of a television image of the 4/3 format displayed on a 16/9 format screen, while performing simultaneous action on the scanning amplitude with the aid of a diode modulator.
2. Description of the Related Art
A horizontal scanning television circuit as described in the opening paragraph is known from the German Patent DE-A-2 655 946. In the circuit described in this document, it is possible to connect an additional S capacitance by means of an electronic switch for adapting the circuit when it is desirable to modify the line frequency (multistandard device). A problem occurs because of the presence of high DC and AC voltages at the two terminals of the S capacitor, while it is generally preferred to use at least a voltage close to ground in an electronic switch. In accordance with this document, this problem is solved by connecting the additional capacitance by means of a transformer.